<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<link rel="STYLESHEET" href="lib.css" type='text/css' />
<link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" />
<link rel='start' href='../index.html' title='Python documentation Index' />
<link rel="first" href="lib.html" title='Python library Reference' />
<link rel='contents' href='contents.html' title="Contents" />
<link rel='index' href='genindex.html' title='Index' />
<link rel='last' href='about.html' title='About this document...' />
<link rel='help' href='about.html' title='About this document...' />
<link rel="next" href="module-gc.html" />
<link rel="prev" href="module-traceback.html" />
<link rel="parent" href="python.html" />
<link rel="next" href="module-gc.html" />
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name='aesop' content='information' />
<title>26.8 __future__ -- Future statement definitions</title>
</head>
<body>
<div class="navigation">
<div id='top-navigation-panel' xml:id='top-navigation-panel'>
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="26.7.1 traceback Example"
  href="traceback-example.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></a></td>
<td class='online-navigation'><a rel="parent" title="26. python Runtime Services"
  href="python.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up one Level' width='32' /></a></td>
<td class='online-navigation'><a rel="next" title="26.9 gc  "
  href="module-gc.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></a></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></a></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></a></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="traceback-example.html">26.7.1 Traceback Example</a>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="python.html">26. Python Runtime Services</a>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="module-gc.html">26.9 gc  </a>
</div>
<hr /></div>
</div>
<!--End of Navigation Panel-->

<h1><a name="SECTION0028800000000000000000">
26.8 <tt class="module">__future__</tt> --
         Future statement definitions</a>
</h1>

<p>
<a name="module-future"></a>

<p>
<tt class="module">__future__</tt> is a real module, and serves three purposes:

<p>

<ul>
<li>To avoid confusing existing tools that analyze import statements
      and expect to find the modules they're importing.

<p>
</li>
<li>To ensure that future_statements run under releases prior to 2.1
      at least yield runtime exceptions (the import of
      <tt class="module">__future__</tt> will fail, because there was no module of
      that name prior to 2.1). 

<p>
</li>
<li>To document when incompatible changes were introduced, and when they
      will be -- or were -- made mandatory.  This is a form of executable
      documentation, and can be inspected programatically via importing
      <tt class="module">__future__</tt> and examining its contents.

<p>
</li>
</ul>

<p>
Each statement in <span class="file">__future__.py</span> is of the form:

<p>
<div class="verbatim"><pre><tt>
 FeatureName = "_Feature(" <var>OptionalRelease</var> "," <var>MandatoryRelease</var> ","
                         <var>CompilerFlag</var> ")"
 </tt></pre></div>

<p>
where, normally, <var>OptionalRelease</var> is less than
<var>MandatoryRelease</var>, and both are 5-tuples of the same form as
<code>sys.version_info</code>:

<p>
<div class="verbatim"><pre>
    (PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
     PY_MINOR_VERSION, # the 1; an int
     PY_MICRO_VERSION, # the 0; an int
     PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
     PY_RELEASE_SERIAL # the 3; an int
    )
</pre></div>

<p>
<var>OptionalRelease</var> records the first release in which the feature
was accepted.

<p>
In the case of a <var>MandatoryRelease</var> that has not yet occurred,
<var>MandatoryRelease</var> predicts the release in which the feature will
become part of the language.

<p>
Else <var>MandatoryRelease</var> records when the feature became part of
the language; in releases at or after that, modules no longer need a
future statement to use the feature in question, but may continue to
use such imports.

<p>
<var>MandatoryRelease</var> may also be <code>None</code>, meaning that a planned
feature got dropped.

<p>
Instances of class <tt class="class">_Feature</tt> have two corresponding methods,
<tt class="method">getOptionalRelease()</tt> and <tt class="method">getMandatoryRelease()</tt>.

<p>
<var>CompilerFlag</var> is the (bitfield) flag that should be passed in the
fourth argument to the builtin function <tt class="function">compile()</tt> to enable
the feature in dynamically compiled code.  This flag is stored in the
<tt class="member">compiler_flag</tt> attribute on <tt class="class">_Feature</tt> instances.

<p>
No feature description will ever be deleted from <tt class="module">__future__</tt>.

<div class="navigation">
<div class='online-navigation'>
<p></p><hr />
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="26.7.1 traceback Example"
  href="traceback-example.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></a></td>
<td class='online-navigation'><a rel="parent" title="26. python Runtime Services"
  href="python.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up one Level' width='32' /></a></td>
<td class='online-navigation'><a rel="next" title="26.9 gc  "
  href="module-gc.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></a></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></a></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></a></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="traceback-example.html">26.7.1 Traceback Example</a>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="python.html">26. Python Runtime Services</a>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="module-gc.html">26.9 gc  </a>
</div>
</div>
<hr />
<span class="release-info">Release 2.5.1, documentation updated on 18th April, 2007.</span>
</div>
<!--End of Navigation Panel-->
<address>
See <i><a href="about.html">About this document...</a></i> for information on suggesting changes.
</address>
</body>
</html>
